A conventional apparatus for punching a metal tape to be a patterned member comprises a die plate for placing a metal tape thereon, a stripper plate for pressing the metal tape on the die plate, punches for punching the metal tape pressed on the die plate by the stripper plate, wherein the punches are held by an upper base moving down and up in operation, and the stripper plate is provided with punch slots and fixed to a stripper holder which is pulled up and resiliently moved down by the upper base, while the die plate is provided with punch slots for allowing the punches to move down and is positioned on a backing plate provided on a lower base.
In operation, a metal tape is placed on the die plate, and the upper base is moved down by receiving an appropriate driving force. In accordance with the moving-down of the upper base, the punches and the stripper holder are moved down, and when the stripper plate becomes contacted with the metal tape placed on the die plate, the stripper holder is stopped, and the upper base moves down by a limited distance determined by a compression amount of a coil spring within a designed gap provided between the upper base and the stripper holder. In accordance with the moving-down of the upper base, the punches protrude through the punch slots of the stripper plate from the stripper holder to punch the metal tape and are inserted into the punch slots of the die plate. Thus, the metal tape is punched with a pattern based on the arrangement of the punches.
Then, the punches, and then the stripper holder are pulled up in accordance with the moving-up of the upper base. At this stage, the stripper plate serves as stripping the punched metal tape off the moving-up punches.
The metal tape is, for instance, as thin as 0.15 mm to provide a lead frame having inner and outer leads used for a semiconductor device, in which the inner leads are connected to electrodes of a semiconductor chip.
In the conventional punching apparatus, however, there is a disadvantage in that deflection and torsion of a patterned member obtained by punching the metal tape occurs especially at tip portions of the pattern, because internal distortion remained in the metal tape represents at the punching time, and processing distortion is additionally generated on the metal tape at the same time. As a result, the patterned member such as a lead frame is deteriorated in precision.
One of reasons why the deflection and the torsion occur is that the die plate is deformed independently of the stripper holder and plate at the punching time, so that the conformity between the stripper plate and the die plate is not sufficiently obtained.